[tvOS] Episode/Season Focus gets Stuck (#1185)

* Focus on the first element if the lastFocusedEpisodeId isn't in the viewModel.

* Performance Improvements:
- Optimized Lookup:  Used a Set for efficient checking of whether lastFocusedEpisodeID exists in viewModel.elements.
- Single Pass:  Reduced the number of passes needed to check for existence and set the focusedEpisodeID.

* https://github.com/jellyfin/Swiftfin/pull/1185#pullrequestreview-2241625398 Suggestions

---------

Co-authored-by: Joseph Kribs <joseph@kribs.net>

Swiftfin tvOS/Views/ItemView/Components/EpisodeSelector/Components/EpisodeHStack.swift

@@ -85,6 +85,11 @@ extension SeriesEpisodeSelector {
                 guard let newValue else { return }
                 lastFocusedEpisodeID = newValue
             }
+            .onChange(of: viewModel.state) { _, newValue in
+                if newValue == .content {
+                    lastFocusedEpisodeID = viewModel.elements.first?.id
+                }
+            }
         }
     }